Single nucleotide variants (SNVs) and phasing
Single nucleotide variants (SNVs) have been widely studied for their associations with phenotypic variation and disease; they are also used to phase haplotypes. However, the use of traditional sequencing technology requires PCR, limiting SNV detection to regions amenable to amplification, and short reads make phasing challenging to resolve. With nanopore sequencing, PCR is not necessary, revealing SNPs in regions inaccessible to other technologies, and phasing is greatly simplified.
Enhance variant calling and phasing with long sequencing reads
Array-based and short-read sequencing analyses of SNVs have shed light on the association between SNVs and the inheritance of complex diseases. However, data from such technologies is only available when a run is completed, which can prevent a rapid response in situations where speed may be vital, such as in outbreak investigations, forensic analysis, and clinical research. Real-time sequencing and analysis with Oxford Nanopore technology provide rapid sample-to-genotype, enabling an immediate response. The technology is also scalable: high-throughput variant calling in larger genomes or in multiplexed samples can be performed with GridION and PromethION; the portable MinION is ideal for variant calling in smaller genomes, and for targeted detection, both in the laboratory and in the field.
Assigning a variant to the maternal or paternal chromosome is important for understanding inheritance patterns, mosaicism, and parental origin of de novo mutations, for example. To directly resolve the haplotype of two heterozygous SNPs, they both need to be present within the same sequencing read. This is inherently challenging with short sequencing reads. Nanopore long reads provide sequence context and enhance the phasing of variants (Figure 1).
Nanopore sequencing enables genome-wide variant calling with high precision and recall
Traditional short-read-based methods for SNV calling require PCR, which can introduce bias, and prevents investigation into genomic regions that are not amenable to amplification. With Oxford Nanopore, native DNA strands can be sequenced, negating the requirement for PCR; this enables greater breadth of genome coverage for SNV calling, as well as access to methylation information from the same sequencing run as standard.
High precision and recall of genomic variants can be obtained from nanopore sequence data. Table 1 displays the latest Oxford Nanopore benchmarking metrics for SNP and indel calling in the human genome, and recommended tools to achieve similar values. The development and optimisation of variant calling tools is a very active area of research. Different bioinformatics tools for SNP calling show variation in performance, and therefore it is important to optimise filtering parameters of the analysis workflow to achieve the best sensitivity and specificity.
Identifying and phasing mutations in the GBA gene using long nanopore sequencing reads
‘…in addition to disease‐causing variants, [the MinION] can detect intronic ones and provide phasing information. The MinION protocol can thus provide further insights into GBA than other sequencing technologies…’
Mutations in the GBA gene, which encodes the lysosomal enzyme glucocerebrosidase, cause Gaucher disease when biallelic, and are strongly associated with Parkinson’s disease. The nearby pseudogene GBAP, which has 96% exonic sequence identity to the GBA coding region, makes analysis of this gene challenging for short-read sequencing due to incorrect mapping.
Leija-Salazar et al. evaluated long-read MinION sequencing of an 8.9 kb amplicon spanning the GBA gene, from 102 individuals. All known missense mutations, including the common p.N409S (N370S) and p.L483P (L444P) mutations, were detected, plus additional rare variants. The team also successfully phased mutations. Any rare false positive SNVs were easily identified and filtered.
The researchers determined that >100x depth of coverage could detect all variants, with no false positives after filtering, although they suggested that a lower depth of coverage (~50x) may be adequate.
How can I call and phase SNVs/SNPs with nanopore sequencing?
To perform genome-wide small variant calling with nanopore technology, we recommend sequencing a whole human genome on the PromethION, or sequencing on the MinION or GridION for targeted analyses. Libraries can be prepared using the Ligation Sequencing Kit, for high throughput and long reads, without amplification.
We recommend Medaka for calling single nucleotide variants and indels. However, other analysis tools are also available from the Community.
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